1. Field of the Invention
The present invention relates to a focus adjusting apparatus for an optical pickup apparatus for use in an optical magnetic disk apparatus or the like, and more particularly to an improvement of the focus adjusting apparatus for concentrating light reflected by a recording medium to two focuses through an optical divider and for positioning a focus adjusting mechanism by outputs of two optical detectors disposed before one focus and after the other focus, respectively.
2. Description of the Prior Art
Various focus adjusting apparatuses for an optical pickup apparatus used in an optical magnetic apparatus or the like are heretofore known, while the present invention is particularly directed to the focus adjusting apparatus which concentrates light reflected by a recording medium to two focuses through an optical divider and positions a focus adjusting mechanism in accordance with quantities of detected light of two optical detectors disposed before one focus and after the other focus, respectively.
Referring to FIGS. 1 and 13, a basic principle of the focus adjusting apparatus to which the present invention is directed is described.
In the specification, a tracking direction, a rotational direction of a disk and a focusing direction are handled as an X axis, a Y axis and a Z axis, respectively.
Referring to FIG. 1, light emitted from a semiconductor laser 1 is collimated by a collimator lens 2 and enters an optical block 3 including beam shaping prisms 3a and 3b and a beam splitter (a boundary G between glasses 3b and 3c serves as the beam splitter).
Generally, since a section of collimated a laser beam shaped by the collimator lens 2 has the elliptical distribution of intensity in accordance with characteristics of the laser beam, the section of the collimated laser beam is corrected into a round section by the beam shaping prism 3a.
More particularly, it is assumed that the major axis of the ellipse is vertical to the paper and when the laser beam enters a plane E of the beam shaping prism 3a made of, for example, material BK7 (BSC7) with an incident angle .theta. and is refracted with a refraction angle .phi., the section of beam is spread in the direction of the minor axis by cos .phi. /cos .theta. and becomes generally round.
Further, a wavelength .lambda. of laser beam generally varies slightly in accordance with the temperature and the intensity of the emitted light and the refraction angle .phi. thereof also varies in accordance with the temperature and the intensity of emitted light.
Thus, the prism 3b made of glass of material SF11 (FD11), for example, having the waveform dependency and the temperature dependency with respect to the refractive index is provided and the laser beam is caused to enter the boundary F between the glasses 3a and 3b with an incident angle .alpha. and to be refracted with a refraction angle .beta. so that the waveform dependency and the temperature dependency of the laser beam are compensated for.
The boundary G between the glasses 3b and 3c constitutes a non-phase beam splitter. 70 to 80 percent of the polarization P of the laser beam are transmitted through the boundary G and most of the polarization S is reflected by the boundary G.
An inclined plane H of 45 degrees is formed at the bottom of the glass 3c and is provided with a coating. The polarization P transmitted through the boundary G enters the inclined plane H with an incident angle of 45 degrees and is totally reflected with non/phase shift. The totally reflected light beam is emitted from the optical block 3 toward the Z axis. The emitted light from the optical block 3 is focused on a disk not shown by an objective lens 4 and the reflected light by the disk is again collimated by the objective lens 4. The collimated light enters the glass 3c and is totally reflected by the inclined plane H to enter the boundary G.
In the case of the optical magnetic disk system, when the beam spot is reflected by a recording plane of the disk, the polarization plane thereof is rotated by a vertical magnetization forming an information pit by a Kerr angle.
Accordingly, when the reflected light by the recording plane of the disk enters the boundary G, 20 to 30 percent of the polarization P and the polarization S are reflected and the reflected light enters a 1/2-wavelength plate 5 so that the polarization plane is inclined by 45 degrees with respect to a beam splitter 7 described later.
Light emitted from the 1/2-wavelength plate 5 passes through a focusing lens 6 into the beam splitter 7 to be divided by the beam splitter 7 into a P component and an S components which are concentrated on two focuses F and F', respectively.
A detector 8 is disposed before the focus F and a detector 9 is disposed after the focus F'. Light receiving planes A and B having an equal diameter are formed in the vicinity of an optical axis of the detectors 8 and 9, respectively. In FIG. 1, sides and fronts of the detectors 8 and 9 are shown.
When spot diameters of light beams S incident on the detectors 8 and 9 are defined as .phi.SA and .phi.SB, respectively, the spot diameters .phi.SA and .phi.SB upon focused exceed the diameter of the light receiving planes A and B sufficiently.
Further, preferably, when quantities of received light of the light receiving planes A and B are defined as a and b, a distance of the detector 8 to the focus F and a distance of the detector 9 to the focus F' are set so that the quantities a and b of received light upon focused are substantially identical.
With such a mechanism, when the disk plane is nearer with respect to the objective lens 4 than the focus point, the reflected light beams are concentrated farther than the focuses F and F'.
Accordingly, in this case, there is a relationship of .phi.SA&gt;.phi.SB between the spot diameters .phi.SA and .phi.SB of the light beams.
This means that the light entering the light receiving plane A is spread as compared with the light entering the light receiving plane B and accordingly there is a relationship of a&lt;b between quantities a and b of detected light of the light receiving plane A and B.
On the other hand, when the disk plane is farther with respect to the objective lens 4 than the focus point, the reflected light beams are concentrated before the focuses F and F'.
Accordingly, in this case, there is a relationship of .phi.SA&lt;.phi.SB between the spot diameters .phi.SA and .phi.SB of the light beams entering the detectors 8 and 9, and since light entering the light receiving plane B is spread as compared with light entering the light receiving plane A, there is a relationship of a&gt;b between quantities a and b of detected light of the light receiving planes A and B.
Accordingly, as shown in FIG. 13, outputs of the light receiving planes A and B are applied to a differential amplifier 12 to drive the objective lens 4 so that a difference a-b of the quantities of detected light of the planes A and B is reduced to 0, to thereby attain focusing. In FIG. 13, numeral 13 denotes a driver including a compensation circuit, and numeral 14a denotes a focusing coil.
As described above, in the focus adjusting apparatus in which the reflected light from the recording medium is distributed onto the two detectors by means of the beam splitter and the servo-system is operated by the difference between the quantities of detected light of the two detectors, it is a precondition for the normal operation that the quantities of detected light of the two detectors are balanced upon focused.
Thus, in order to balance the quantities of detected light of the two detectors 8 and 9, generally, positioning of the 1/2-wavelength plate 5 is made, while there is a problem that the positioning of the 1/2-wavelength plate 5 requires extremely accurate adjustment.
Further, even when the positioning of the 1/2-wavelength plate 5 is made accurately, unbalance between the quantities of detected light of the two detectors occurring by influence of crosstalk between tracks and double refraction on a disk plane can not be treated by a conventional system and there is no guarantee that focusing accuracy can not be always ensured.